1. Field of the Invention
The present invention relates to a method for forming two intermediate frequency signals in different phases in a mixer, to which a first and a second signal are conducted, a first intermediate frequency signal is formed by shifting the phase of the first signal and by mixing the phase-shifted first signal and the second signal, and a second intermediate frequency signal is formed by mixing the first and the second signal. The invention also relates to a mixer, which comprises at least means for conducting the first signal to the mixer, means for conducting the second signal to the mixer, means for forming a first intermediate frequency signal of the first signal, the phase of which has been shifted, and the second signal, and means for forming a second intermediate frequency signal of the first and the second signal. The invention also relates to a receiver, which comprises means for receiving the first signal, which has been modulated in the transmission step, means for forming a second signal, a mixer, means for conducting the first signal to the mixer, means for conducting the second signal to the mixer, means for forming a first intermediate frequency signal of the first signal, the phase of which has been shifted, and the second signal, and means for forming a second intermediate frequency signal of the first and the second signal.
2. Brief Description of Related Invention
In some prior art receivers intended for the reception of radio signals, the received high-frequency radio signal is converted to one or more intermediate frequencies before the information transmitted in the signal is separated from it. Each intermediate frequency is typically lower than the received radio frequency signal. Conversion to the intermediate frequency is generally performed in a mixer, where a mixing frequency generated preferably by a local oscillator, also called a local oscillator signal, is mixed in the receiver with the radio frequency signal. The mixing result provides two signals in the output of the mixer, wherein the frequency of the first signal is the difference between the frequency of the received radio signal and the local oscillator frequency, and the frequency of the second signal is the sum or the frequency of the received radio signal and the local oscillator frequency. The required intermediate frequency signal, typically the difference signal mentioned above, can be separated from these signals by conducting the output signal of the mixer to the band-pass filter. The passband of this band-pass filter has been set so that the desired frequency range passes the band-pass filter essentially unattenuated. Other frequencies, such as the above mentioned sum signal, cannot pass this band-pass filter in any significant amount. A solution like this operates satisfactorily when the intermediate frequency used is relatively high in comparison to the frequency range of the signal to be separated. In audio applications, such as broadcast receivers, the frequency range of the signal to be separated is in the range 20 Hz–20 kHz. In mobile stations, this frequency range may be somewhat smaller, e.g. 300 Hz–4 kHz, while the sound quality is still good enough for transmitting speech.
In broadcast receivers, the intermediate frequency is typically set at about 10.7 MHz in FM applications and at about 455 kHz in AM applications. Recently, however, there has been a tendency especially in portable devices, such as mobile stations, to reduce the size and power consumption of the devices. This has been implemented by increasing the degree of integration of the devices, whereby high frequency and intermediate frequency circuits, for instance, have been placed on an integrated circuit. However, this sets limitations to the implementation of intermediate frequency stages, for instance. The degree of integration can be increased in a radio receiver, if the receiver is implemented by using direct conversion, in which case no intermediate frequency stages are needed, or so that the intermediate frequency used is as low as possible due to interference and other such factors (low-IF).
The implementation of a direct conversion receiver is relatively simple, but the problem is the control of DC offset in the separated signal. On the other hand, the use of a low intermediate frequency makes the attenuation of so-called image frequency signals more difficult. Image frequency signals consist of other radio signals, which are strong and close to the frequency being listened to. These radio signals reach the mixer of the receiver relatively unattenuated, and when mixed with the local oscillator signal, the signals formed as the mixing result are in a frequency range, which is at least partly within the frequency range of the intermediate frequency signals formed in the reception of the desired radio signal, or close to it. When a relatively high intermediate frequency is used, the image frequency signals can be attenuated by using an image frequency filter, such as a band-stop filter tuned to a suitable frequency. On the other hand, when low intermediate frequencies are used, it is not possible in practice to separate the image frequency signals from the desired signal in the band-pass filter, because its Q value should be unrealistically high in order to operate efficiently enough. When the band (BW) of the band-pass filter is wider than the intermediate frequency, the image frequency cannot be separated by filtering. For example, if the band of the band-pass filter is about 78 MHz and the intermediate frequency is about 3 MHz, it is not possible in practice to separate the image frequency by filtering.
There are some prior art solutions for intensifying the image frequency attenuation in receiver applications, in which a relatively low intermediate frequency, e.g. in the range of some tens of kilohertz, is used. Examples of these solutions that may be mentioned in this connection are the Weaver and Hartley topologies. The idea in these is to attenuate the image frequencies in connection with the mixer, and thus a separate image frequency filter is not needed. A prior art Hartley mixer is shown in FIG. 1. It comprises, among other things, two mixer blocks, whereby the received radio frequency signal RF is mixed in the first mixer block 2 with a local oscillator signal LO, on which a 90° phase shift has been performed in the first phase shift block 4 and which has been amplified in the first buffer stage 6. In the second mixer block 3, the received radio frequency signal is mixed without phase shift with the local oscillator signal LO, which has been amplified in the second buffer stage 7. In addition, a 90° phase shift is performed on the mixing result of the second mixer block in the second phase shift block 5. There are two intermediate frequency signals in the output of the mixer, and these are summed in the adder 8 located after the mixers. It can be proved mathematically that the phase shift has a different effect on the desired frequency than the image frequency, and thus in an ideal case the desired signal has the same phase in summing and is sustained, whereas the image frequency has the opposite phase and is eliminated in a Hartley mixer. In addition, the intermediate frequency signal is demodulated in order to detect the transmitted information.
A prior art Hilbert mixer is shown in FIG. 2. The main difference between this Hilbert mixer and the above described Hartley mixer is the fact that phase shift is not performed on the local oscillator signal before conducting it to the second mixer. Instead, phase shift is performed on the received radio frequency signal in the first phase shift block 4 before conducting the signal to the second mixer block of the mixer. There are two intermediate frequency signals in the output of the mixer, and these signals are summed in the adder 8 located after the mixers. The fact that the phase shift has a different effect on the desired frequency than on the image frequency is also utilized in this mixer solution, and thus in an ideal case the desired signal has the same phase in summing and is sustained, whereas the image frequency has the opposite phase and is eliminated in a Hilbert mixer. In addition, the intermediate frequency signal is demodulated in order to detect the transmitted information.
However, a drawback of the mixer solutions described above is the fact that in order to accomplish sufficiently reliable operation of the mixers, the components used in the mixers must be accurately measured and matched to each other. Thus the manufacturing tolerances of the components must be very small, and this causes considerable difficulties in practical applications and increases the price of the receivers.
It is an object of the present invention to accomplish a method for converting a radio signal to an intermediate frequency in a mixer, and a mixer and a receiver. The invention is based on the idea that phase shift is performed on the received radio frequency signal in the mixer and not before it. The method according to the present invention is characterized in that the phase shift of the first signal is performed in the mixer. The mixer according to the present invention is characterized in that the mixer also comprises at least means for performing phase shift on the first signal. In addition, the receiver according to the present invention is characterized in that the mixer also comprises at least means for performing phase shift on the first signal.